Geophysical and geological observations document that beneath the submerged forearc, processes of sediment subduction and subduction erosion move large volumes of material toward the mantle. The conveying system is the subduction channel separating the upper plate from the underthrusting ocean plate. Globally, the zero-porosity or solid-volume rate at which continental debris is shuttled toward the mantle is estimated to be ∼2.5 km 3 /yr. To deliver this volume, the average thickness of the subduction channel is ∼1.0 km. Some deeply subducted material is returned to the surface of Earth as a component of arc magma or as tracks of high- P/T crustal underplates. But over long periods of time (>50 m.y.), most of the removed material is evidently recycled to the mantle. Applying Cenozoic recycling rates to the past astonishingly implies that since 2.5 Ga a volume of continental crust equal to the standing inventory of ∼6 × 10 9 km 3 has been removed from the surface of Earth. This minimum estimate does not include crustal material recycled at continental collision zones nor reliable estimates of recycling where large accretionary bodies form. The volume of demolished crust is so large that recycling must have been a major factor determining the areal pattern and age distribution of continental crust. The small areal exposure of Archean rock is thus probably more a consequence of long-term crustal survival than the volume originally produced. Reconstruction of older supercontinents is made difficult if not unachievable by the progressive truncation of continental edges effected by subduction zone recycling, in particular by subduction erosion.

New 40 Ar/ 39 Ar, major and trace element, and isotopic data for ca. 24–15 Ma backarc volcanic rocks from the Sierra de Huantraico, Sierra Negra, and Sierra de Chachahuén–La Matancilla regions (36°S–38°S) in the Neuquén Basin shed light on the early Miocene evolution of the south-central Andes. A model calling for incipient shallowing of the sub-ducting slab under the northern Neuquén Basin and an increase in the rate of westward motion of South America relative to the underlying mantle at ca. 20 Ma can explain many regional features. Early Miocene magmatism in the Neuquén Basin began with the eruption of ca. 24–20 Ma alkali olivine basalts from monogenetic and simple polygenetic centers located up to 500 km east of the trench. Their characteristics (Ta/Hf > 0.45, ε Nd = +3.6–+4.2; La/Ta < 14; Ba/La < 16; 87 Sr/ 86 Sr = 0.7037–0.7040) indicate a backarc mantle devoid of arc-like components. These basalts erupted at a time of extension all along the margin during a period of rapid, near-normal Nazca–South America plate convergence when spreading ridges between the Pacific, Nazca, and Antarctic plates were becoming more parallel to the Chile Trench. Ridge rotation along with slab roll-back in response to slow relative motion between South America and the underlying mantle can explain why isotopically enriched magmas erupted far to the east of the trench in a generally extensional regime. Subsequently, 19–15 Ma basaltic to trachyandesitic backarc lavas with weak arc-like La/Ta (15–26), Ba/La (15–32), and Ta/Hf (0.2–4.5) ratios and a more depleted isotopic signature (ε Nd = +3.9–+4.7; 87Sr/86Sr = 0.7033–0.7037) erupted in a contractional regime. Their chemical features fit with incipient shallowing of the Nazca plate under the northern Neuquén Basin. A contractional regime that extended all along the margin can be explained by westward acceleration of South America over the underlying mantle as Nazca–South America plate convergence slowed.

The evolving chemistry of the Chachahuén volcanic complex provides evidence for transient entry of a subduction zone component into the mantle wedge over a late Miocene shallow subduction zone under the Neuquén Basin. The Chachahuén complex, which is in the backarc of the Andean Southern Volcanic Zone near 37°S and some 500 km east of the Chile Trench, occurs at the intersection of NE and SE fault systems that parallel regional trends. Support for a shallow subduction-zone setting at the time of eruption and during the contractional uplift of the Sierra de Chachahuén comes from K/Ar and new 40 Ar/ 39 Ar ages, mineral assemblages, major and trace element chemistry, and Nd-Sr-Pb isotopic compositions. Importantly, the chemistry of the Chachahuén rocks requires an arc-like component in the mantle that is absent in both early Miocene or Pliocene alkaline lavas erupted in the same region. The oldest Chachahuén volcanic rocks are the ca. 7.3–6.8 Ma Vizcachas group orthopyroxene-bearing andesites to rhyodacites that erupted from fissures and small centers along the NE-trending fault system. Intraplate chemical tendencies in the most silicic samples are attributed to mantle-derived basalts interacting with a lower crust that has a chemical imprint that reflects older alkaline magmatic events. Younger Chachahuén group volcanic rocks erupted at ca. 6.8–6.4 Ma from vents generally aligned along the NE-trending fault system and ca. 6.3–4.9 Ma magmas that erupted from a trap-door–type caldera and flanking stratovolcanoes along the NW-trending fault system. These high-K basaltic to dacitic rocks contain amphibole phenocrysts and show arc-like high field strength element depletions that are the strongest in basaltic andesite lavas. Parallels between Chachahuén volcanic rocks and uplift of the Sierra de Chachahuén with late Miocene Pocho volcanic rocks and uplift of the Pampean Ranges over the modern Chilean flat-slab support transient Miocene shallow subduction zone under the Neuquén Basin.